Stop adding xcframework Headers to search paths when a module map is present

Targets depending on a precompiled xcframework that ships a module.modulemap no longer fail with “redefinition of module ‘X’”. Xcode’s ProcessXCFramework already extracts the SDK-matching slice’s headers into the build’s include directory, so the duplicate Headers directory added for cached library products is now skipped when the xcframework carries a module map. Module-map-less xcframeworks (the cached library/Swift products) keep their headers.